Benzoxazine-based coatings for improving corrosion performance of anodized aluminum alloys

Aluminum alloys are widely used in aircraft applications, especially series 2xxx (Cu) and 7xxx (Zn). Those alloys offer improved mechanical properties compared to pure aluminum, but are strongly sensitive to corrosion. Chromic anodizing was used as an efficient corrosion protection. But because of health and environmental issues, this treatment is going to be forbidden. Alternative solutions have to be developed to reach aircraft standards considering health and environmental concerns.
The purpose of this work is to develop an anti-corrosion system consisting in a cross-linked resin coated on an oxide film obtained by sulfotartric anodizing (TSA) process. The role of the anodic film is to improve compatibility between coating and substrate by providing a great interaction surface to the resin thanks to the porous structure of the oxide layer.
We investigated the influence of anodizing parameters (voltage, time, bath temperature) in order to control the anodic film properties (pore size, porosity and thickness). Oxide layers were observed and characterized using Field Emission Gun Scanning Electron Microscopy (FEG-SEM). An organic coating based on benzoxazines, a new class of thermoset resins offering low shrinkage and high chemical resistance was used. We have studied the thermal crosslinking of such resins using Dielectric Analysis (DEA), Differential Scanning Calorimetry (DSC) and IR spectroscopy.
Coatings were applied by spin coating on either bare or anodized clad Al-2024 substrates and characterized using Electrochemical Impedance Spectroscopy (EIS) to emphasize its anti-corrosion behaviour. The results confirmed the strong potential of such resins as anti-corrosion solutions.